The present invention generally relates to satellite communication systems, and more particularly to apparatus and methods for coupling a satellite to an earth terminal.
Satellite-based communication systems have continued to evolve and have become an important component of modern society. Numerous applications are supported by satellite-based communication systems that provide wide area coverage, such as worldwide television, communications to remote areas, wide area data networks, global personal communications to hand-held portable telephones, broadband voice, video, and/or data. As the number of applications supported by satellite-based communication systems has increased and the number of users using the applications has increased, processes have been developed to accommodate the increased number of applications and users.
One process that has been developed to accommodate the increased number of applications and users is multiplexing. The process of multiplexing allows multiple signals to be sent on a single channel, and many forms of multiplexing have been developed to generate a multiplexed communication signal, including, but not limited to time multiplexing, frequency multiplexing, space multiplexing (e.g., Frequency-Division Multiplexing (FDM), Time-Division Multiplexing (TDM), Space-Division Multiplexing (SDM), Orthogonal Frequency Multiplexing (OFM), Code-Division Multiple Access (CDMA) multiplexing, Wideband CDMA (WCDMA) multiplexing, Time-Division Multiple Access multiplexing, Orthogonal Frequency Multiple Access (OFMA) multiplexing, and Frequency Division Multiple Access multiplexing (FDMA)). While multiplexing has increased the number of signals that can be sent on a single channel to accommodate numerous applications and users, additional complications are introduced when multiplexing is utilized in satellite-based communication systems.
For example, existing and proposed satellite-based communication systems (e.g., wideband CDMA Mobile Satellite Systems (MSS)) use an analog, frequency division multiplexed (FDM) channelized approach that maps each uplink beam and carrier, which is typically an L or S band carrier, to a specific gateway downlink frequency, which is typically a C, X or Ka band carrier. This analog FDM channelized approach directly couples the user uplink to the earth terminal downlink, so that fading or other power variation in the earth terminal downlink results in uncontrolled power variations in the measurement of the user uplink, thereby degrading the ability to accurately set the user uplink power level. In addition, the use of the analog FDM channelized approach introduces nonlinearities, which generally results in an increase in the noise floor due to intermodulation distortion, such that the presence of a few high capacity, and hence high power beams, will undesirably increase the noise power of other beams and channels. Also, a large external interfere in one beam can also undersirably increase the noise in other beams and carriers due to this nonlinearity. Furthermore, the introduction of nonlinearities will also generally degrade power control as large carrier codes will be affected to a greater extent than small carrier codes due to a tendency for the large carrier codes to become compressed prior to the small carrier codes. As can be appreciated by those of ordinary skill in the art, the foregoing effects reduce the likelihood of implementing numerous tasks, such as digital beam forming, on the satellite as the relative amplitude and phase characteristics between channels, corresponding to different feed elements, are altered in a unpredictable manner.
To overcome issues such as power control and linearity, processing on-board the satellite has been proposed to generate the multiplexing signal (e.g., CDMA signal). However, increasing the processing on-board the satellite typically increases the cost of the satellite. Furthermore, the initial configuration of the satellite to support a particular multiplexing waveform (e.g., CDMA signal) is generally unalterable during the lifetime of the satellite.
In view of the foregoing, it should be appreciated that it would be desirable to provide apparatus and methods for coupling a satellite to an earth terminal that addresses the foregoing and other deficiencies of satellite-based communication systems that are not specifically or inferentially addressed in this background of the invention. Furthermore, additional desirable features provided by the invention will become apparent to one skilled in the art from the drawings, foregoing background of the invention, following detailed description of the drawings and appended claims.